Malaria currently represents one of the most prevalent infections in tropical and subtropical areas throughout the world. Per year, malaria infections kill up to 2.7 million people in developing and emerging countries. The widespread occurrence and elevated incidence of malaria are a consequence of the increasing numbers of drug-resistant parasites and insecticide-resistant parasite vectors. Other factors include environmental and climatic changes, civil disturbances and increased mobility of populations.
Malaria is caused by mosquito-borne hematoprotozoan parasites belonging to the genus Plasmodium. Four species of Plasmodium protozoa (P. falciparum, P. vivax, P. ovale and P. malariae) are responsible for the disease in man; many others cause disease in animals, such as P. yoelii, P. knowlesi and P. berghei. P. falciparum accounts for the majority of infections in humans and is the most lethal type.
The two major arms of the pathogen-specific immune response that occur upon entry of the parasite into the body are cellular and humoral. The one arm, the cellular response, relates to CD8+ and CD4+ T cells that participate in the immune response. Cytotoxic T lymphocytes (CTLs) express CD8 and are able to specifically kill infected cells that express pathogenic antigens on their surface. CD4+ T cells or T helper cells support the development of CTLs, produce various cytokines, and also help induce B cells to divide and produce antibodies specific for the antigens. During the humoral response, B cells specific for a particular antigen become activated, replicate, differentiate and produce antigen-specific antibodies.
Both arms of the immune response are relevant for protection against a malarial infection. A possible malaria vaccine approach would be most beneficial if it would induce a strong cellular immune response as well as a strong humoral immune response. It is widely accepted that persistent protective immunity against malaria likely requires high levels of Th1 type immune responses targeting the pre-erythrocytic stage of the malaria parasites.
However, almost forty years after the feasibility of vaccination against malaria was first demonstrated by means of irradiated sporozoites, a vaccine modality that efficiently induces long-lived protective immunity remains elusive. The most advanced CS-based malaria vaccine candidate to date is RTS,S, a vaccine based on a fragment of Plasmodium falciparum circumsporozoite (CS) protein, fused to and admixed with hepatitis B surface protein. This vaccine confers short-term protection against malaria infection with an efficacy of about 50% and induces particularly B-cell and CD4+ T-cell responses.
Albeit our understanding about the correlate(s) of protection for malaria is limited, there is ample evidence that circumsporozoite (CS) protein-specific antibodies, CD8+ T cells and Th1 cytokines, and, in particular, IFNγ, play a central role in controlling the pre-erythrocytic and early liver stages of malaria. Adenoviral vectors appear particularly suited for induction of IFNγ-producing CD8+ T cells required to combat malaria infection (Ophorst et al., 2006; Rodrigues et al., 1997), due to intracellular expression of a transgene inserted in the vector genome and efficient routing of expressed protein toward the class I presentation pathway.
WO 2006/040334 describes prime boost regimens for malaria vaccination by administering a replication-defective recombinant adenovirus comprising nucleic acid encoding a CS antigen from a malaria-causing parasite and further administering adjuvanted proteinaceous antigen comprising a CS protein or immunogenic fragment thereof, and, amongst many other possibilities, describes adenovirus serotype 35 (Ad35) and Ad26 as preferred adenoviruses. WO 2006/040334 teaches that priming with the viral vector and boosting with the proteinaceous antigen provides superior results in terms of immune responses compared to the reverse regimen, in particular, with respect to IFN-γ T-cell responses. A particularly preferred regimen described therein comprises priming with Ad35 encoding a P. falciparum CS antigen and boosting twice with RTS,S. Indeed, this regimen is also demonstrated to be superior to the regimen wherein the order of administration of the protein and adenovirus are reversed in the article by Stewart et al., 2007. Thus, Ad35 with a CS antigen appears to be a very suitable priming vaccine for boosting by CS protein.
Antibody as well as robust IFN-γ responses against the CS antigen have also been reported upon a heterologous prime boost vaccination schedule wherein Ad35 encoding CS was boosted with Ad5 encoding CS (Rodriguez et al., 2009).
It has also been shown for the LSA-1 antigen that Ad35 priming followed by protein boosting results in induction of IFN-γ producing CD4+ and CD8+ T cells, although it could also be seen that the types of immune responses might differ between different transgenes, e.g., CS antigen may behave different from, e.g., LSA-1 antigen (Rodriguez et al., 2008).
In the experience of the inventors, several other prime boost regimens may be different regarding the level of immune responses depending on the antigen, and/or directionality of the prime-boost with respect to the vector used (see also, e.g., Abbink et al., 2007).
Unpredictability of immune responses with respect to different antigens is further underscored by the observation that recombinant BCG (rBCG) expressing P. falciparum CS protein neither resulted in detectable CS responses when administered alone, nor primed CS responses in a prime-boost schedule with Ad35 expressing CS (unpublished), whereas, in contrast, another antigen cloned in rBCG could be boosted by subsequent administration of an adenoviral vector with the same transgene and the same has been shown for vaccination against tuberculosis (TB) by BCG followed by heterologous booster constructs containing TB transgenes (see Cayabyab et al., 2009, and references therein).
Thus, the level and type of immune responses upon vaccination is complex and not fully predictable because it may differ for different transgenes and depend on the type of antigen and administration regimen.
In addition, the most preferred regimen known to date requires Ad35 with a CS antigen followed by two boosts with RTS,S (WO 2006/040334; Stewart et al., 2007). Production of RTS,S (adjuvanted protein) is much more expensive than production of adenoviral vectors, and addition of adjuvant is by definition related to a possibility of more (local) side effects, as is known to the skilled person. Thus, administration regimens requiring less adjuvanted protein such as RTS,S while still being capable of strong immune responses would be beneficial.